Despite their youth and inexperience, Goten and Trunks inexplicably became Super Saiyans in Dragon Ball Z. This predictably brought about a lot of questions, considering that a seven-year-old Goten and an eight-year-old Trunks were able to pull off a transformation that Goku himself wasn’t able to do until he was well into his 20s.
Following the Cell Saga, a time jump pushed the franchise several years into the future and introduced audiences to a grown-up Gohan, Goku’s second son Goten, and a young version of Trunks. Much to his astonishment, Gohan discovered that Goten had secretly developed the ability to go Super Saiyan. Trunks was able to attain this form as well. Since each of them possessed such an amazing degree of power, Goku and Piccolo took steps to help them prepare for a fight with the saga's villain, Majin Buu. After fusing into Gotenks, their strength was great enough for them to access the Super Saiyan 3 transformation.
How Goten and Trunks were able to become so strong in such a short amount of time is something that’s become a point of confusion for fans. Some have complained that the decision to make Goten and Trunks Super Saiyans cheapened the transformation, since both Goku and Vegeta had to go through several trials just to get to that level. However, Dragon Ball creator Akira Toriyama offered an explanation as to how they were able to pull it off. Part of the reason stems from a concept called “S-Cells”, which is never mentioned in the show [via Twitter].
Toriyama says S-Cells are critical to a Saiyan’s ability to go Super Saiyan. If a Saiyan doesn’t have enough S-Cells, he’ll never be able to reach that state, no matter how hard he trains. A person can increase their S-Cells by having “a gentle spirit”. Apparently, Goten and Trunks inherited a great deal of S-Cells from their fathers. Presumably, they received even more than Gohan did since he was 9 when he went Super Saiyan for the first time. That’s only a one-year difference, but Gohan had an advantage in experience. By the time he reached that age, he had already survived grueling battles with Nappa, Vegeta, Recoome, and Frieza. Goten, on the other hand, had enjoyed a comparably peaceful existence.
Another reason why Goten and Trunks gained this form at a such a young age is the gravity. Toriyama claims that the Earth’s environment is “gentler and easier” to live in than the Saiyan homeworld, Planet Vegeta. It’s true that Planet Vegeta has much stronger gravity than Earth and isn’t a very hospitable place. Gohan benefited from this as well, but again, he may not have had as many S-Cells as Goten and Trunks did. These reasons help to make sense of how they were able to become so powerful so fast, even if many still feel that their Super Saiyan moments weren’t properly earned. For a time, the Buu Saga made them the Z-Warriors’ best hope, but by the time Dragon Ball Super rolled around, it seemed that Goten and Trunks were no longer the Saiyan progenies that the Buu Saga had made them out to be. Neither of the two characters have made any obvious gains since Dragon Ball Z, so it may be a while before one of them hits Super Saiyan 2.